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1972_911T 07-30-2005 07:43 AM

Stock 72t Compression Values
What sort of compression value would you expect to get from a stock 911 t with 7.5 ratio?

If I remember back to my physics lessons atmospheric preassure is 1 bar/ atmosphere at sea level! Therefore a compression ratio of 7.5:1 should give a value of around 7 bar if the engine is in good health of cause. As you are increasing atmospheric pressure by a factor of 7.5

Please feel free to correct me if my assumptions are wrong!

I have just compression tested my engine and it gave values of between 11.2 - 11.8 bar this seems high for a stock 911t. Could it be that my engine is not stock as far as compression goes or am I way out with my assumptions and these figures are what you would expect from a 911 t with 7.5:1 compression ratio?


Shuie 07-30-2005 12:08 PM

the compression test for my '72 2.4T was 124-130 psi on all cylinders. as long as its a reasonable number thats consistent across all cylinders I wouldnt worry too much.

1972_911T 07-30-2005 12:33 PM

Thats interesting cos 11 bar is 160psi, wounder if it is just due to variations in how I performed the test or if my car has uprated compression! Pretty sure its been rebuilt at some point as it has carrera chain tensioners fitted and the oil return pipes still have part number stickers on them dont think they would survive after 30 years


BK911 07-31-2005 10:09 AM

Compression ratio is sort of a misnomer. Its a volume ratio, not a pressure ratio. So 7.5:1 means the volume decreases by 7.5 Xs, not that the pressure increases by 7.5 Xs. The latter would have been correct if the air/fuel mixture stayed the same temperature throughout the compression process. But since the temperature increases during the compression process, it raises the resulting pressure.

1972_911T 07-31-2005 11:21 AM

So basically without tearing the engine down to see what pistons etc its got in it there is no way to tell if it is running a higher compression ratio?


911pcars 07-31-2005 01:26 PM

Well, you could, but it'd be messy. You would have to measure the combustion chamber volume with a cylinder at TDC. Poised with that figure, you can plug in stroke and cylinder volume (if known) and arrive at a pretty close number.

The problem is how to measure CC volume. You would have to fill the cylinder with some liquid through the spark plug opening w/valves closed and make sure all the air is vented for an accurate reading. Doing this in the car is difficult, if not impossible. You'd have a better chance with the engine on a stand.

I'm sure the NASCAR tech guys have a rig to do this conveniently (i.e. fill test cylinder at BDC with test liquid. Measure fluid in, then rotate engine to TDC and measure exiting fluid into graduated beaker. The difference is CC volume, or something like that). Their cylinders are a little more vertical than ours. Any stock car tech guys out there?


BK911 08-02-2005 06:46 PM

I doubt somebody just changed the pistons or cylinders. Look at your cylinders. If they are cast iron (rusty) then you probably have the stock T pistons. If not, then you probably have a higher dome piston. Unless of course they used 2.2T pistons/cylinders.

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